Communications interface device for receiving digital signals

ABSTRACT

A communications interface device ( 4 ) has circuitry for receiving digital signals in the form of characters according to a protocol defining that a communications link is initialized by a series of start characters, each having a specified duration. The receiving circuitry can assume a first state in which characters can be detected, and a second state in which detection of characters is not possible. Power consumption is higher in the first state than in the second state. The device can alternate the receiving circuitry periodically between the first state and the second state with a period shorter than the duration of the series of start characters. In each period the receiving circuitry assumes the first state for a time longer than the duration of a start character. A communications link is established if a character is detected. The power consumption can be reduced considerably without causing unnecessary inconvenience and time consumption for the user.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/272,042, filed Mar. 1, 2001, which is hereby incorporated herein byreference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method of establishing a communications linkfor exchanging digital signals in the form of characters between atransmitter and a receiver, wherein the transmitter for initializationof a communications link transmits a series of start characters. Theinvention further relates to a communications interface device.

DESCRIPTION OF RELATED ART

Short-range communications systems are often used for communicationbetween two (or more) devices located within a short distance from eachother. Examples of such systems are infrared interfaces, short-rangeradio interfaces, such as a Bluetooth interface, or wired connections,and examples of the use of such systems include the communicationbetween a mobile telephone and a portable computer, the communicationbetween a personal computer and a printer, or the communication betweena mobile telephone and an auxiliary equipment, such as a digital cameraor an external memory device.

In such systems there will usually be long periods of time in whichthere is no actual exchange of information to or from a given device,either because there is no other device located within the range of thedevice, or because there is no information to be exchanged. Forinstance, in the communication between a personal computer and aprinter, information is exchanged only when a document is being printed.

However, the receiver part of an interface must be switched on more orless all the time, and thus consumes power, although it is only utilizeda small fraction of the time. For instance, in the above mentionedcomputer/printer example, the printer has no possibility of predictingwhen the user of the computer intends to print a document, and thus thereceiver interface of the printer must be ready for receivinginformation as long as the computer is in use.

A switched-on interface which is not in the process of exchanginginformation can typically be in a primary mode or in a secondary mode.In the primary mode the interface actively searches for other interfacesof the same type, normally because it has information to transmit. Aninterface in the primary mode will typically search for other interfaceswith intervals and transmit in each interval a series of startcharacters. The start characters do not contain any information thatneeds to be propagated or stored in the receiving device, but are merelyused to help the receiver initiate itself for receiving actual data. Inthe secondary mode the interface only listens for an interface inprimary mode to contact it, and, as mentioned above, an interface willtypically be in the secondary mode most of the time. However, even inthe secondary mode the interface has a considerable power consumption.

Especially in portable and battery powered devices the power consumptionis an important issue, because it affects the stand-by time or operatingtime of the device between re-chargings of the battery, and thus it is adrawback that the receiver interface has to be switched on all the time.Of course the receiver interface can be switched on and off manually bythe user of the devices so that the receiver only consumes power when itis needed for reception of information, but this solution will normallybe inconvenient and time consuming for the user, who may have to switchthe receiver interface on and off frequently, and further, the user doesnot always have sufficient knowledge about when information is going tobe exchanged.

WO98/41001 discloses a system in which an interface between a mobiletelephone and a PC (personal computer) is accomplished via an infraredlink. When the telephone is in a passive standby mode, pressing acertain sequence of keys will start the infrared transceiver of thetelephone to monitor or poll for incoming infrared signals with a viewto establishing an infrared link between the telephone and the PC. Ifduring a 20-second period signals seeking to create an infrared link arereceived, such a link is established. If, however, no link isestablished after the 20-second period, the telephone will discontinuemonitoring the incoming infrared signals. Although this system has thepossibility of switching off the receiver automatically, if no link isestablished during the 20-second period, the user still has to switch iton manually, and he even has to switch it on again, if expected signalsfrom the PC did not show up within the first period. Thus this systemalso suffers from the problems mentioned above.

Therefore, it is an object of the invention to provide a method of theabove-mentioned type in which the power consumption can be reducedconsiderably without causing unnecessary inconvenience and timeconsumption for the user.

SUMMARY

According to the invention the object is achieved in that the methodcomprises the steps of controlling the receiver to alternateperiodically between a first state in which characters can be detected,and a second state in which detection of characters is not possible,with a period shorter than the duration of the series of startcharacters, the receiver having a power consumption which is higher inthe first state than in the second state, and the alternation beingcontrolled such that in each period the receiver assumes the first statefor a time which is longer than the duration of a start character; andestablishing a communications link if a character is detected.

By allowing the receiver to be in a receiving state for only a fractionof each period, said fraction being sufficiently large, and the periodbeing sufficiently short, to detect a start character sent by atransmitter, and allowing the receiver to be in a less power consumingstate the rest of the time, the power consumption of the receiver can bereduced considerably, while it is still ensured that any start characteris detected.

When the method further comprises the step of keeping the receivingcircuitry in the first state for a longer time when a communicationslink is established, it is ensured that after a detected start characterthe receiver remains in the first state so that the following datacharacters can also be detected.

The digital signals can be transmitted via a wired connection, via aradio channel, via an infrared channel, or as other optical signals.

When the digital signals are transmitted via an infrared channel, theymay be transmitted according to the IrDA protocol, and they may betransmitted with a data rate of 9600 bit/s. The start character may bean XBOF character. The method may be used in a mobile telephone.

As mentioned, the invention also relates to a communications interfacedevice having receiving circuitry for receiving digital signals in theform of characters from an external transmitter according to a protocoldefining that a communications link is to be initialized by transmittinga series of at least a prescribed number of start characters, each startcharacter having a specified duration; said receiving circuitry beingable to assume a first state, in which characters can be detected, and asecond state, in which detection of characters is not possible; and saidreceiving circuitry having a power consumption which is higher in thefirst state than in the second state.

When the device comprises control means adapted to alternate thereceiving circuitry periodically between the first state and the secondstate with a period shorter than the duration of the series of startcharacters, such that in each period the receiving circuitry assumes thefirst state for a time which is longer than the duration of a startcharacter; and the control means is further adapted to establish acommunications link if a character is detected, the power consumptioncan be reduced considerably without causing unnecessary inconvenienceand time consumption for the user. By allowing the receiver to be in areceiving state for only a fraction of each period, said fraction beingsufficiently large, and the period being sufficiently short, to detect astart character sent by a transmitter, and allowing the receiver to bein a less power consuming state the rest of the time, the powerconsumption of the receiver can be reduced considerably, while it isstill ensured that any start character is detected.

When the control means is further adapted to keep the receivingcircuitry in the first state for a longer time when a communicationslink is established, it is ensured that after a detected start characterthe receiver remains in the first state so that the following datacharacters can also be detected.

The receiving circuitry may be adapted to receive the digital signals aselectrical signals on a wired connection, or the receiving circuitry maycomprise a receiver for radio frequency signals or a receiver forinfrared signals.

When the receiving circuitry comprises a receiver for infrared signals,the receiving circuitry may be arranged to communicate according to theIrDA protocol, and it may be arranged to communicate with a data rate of9600 bit/s. The start character may be an XBOF character.

When the device further comprises transmitter circuitry, the sameinterface device may be used for reception as well as transmission ofdigital signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described more fully below with reference tothe drawings, in which

FIG. 1 shows a mobile telephone connected to a personal computer with aconnection wherein the invention can be utilized,

FIG. 2 shows the configuration of an infrared transceiver in which theinvention can be utilized,

FIG. 3 shows a discovery frame according to the IrDA protocol with theXBOF character having the value 0xFF,

FIG. 4 shows a discovery frame according to the IrDA protocol with theXBOF character having the value 0xC0, and

FIG. 5 shows an example of the signals in a circuit according to theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a situation in which the invention can be utilized. Amobile telephone 1 is connected to a personal computer 2, which is shownin the figure as a laptop computer, via a communications link 3. Theactual communication takes place between a transceiver 4 arranged on themobile telephone 1 and a transceiver 5 arranged on the computer 2. Thetype of the communications link 3 can be selected from several differenttypes, such as a wired connection, a short-range radio link or aninfrared link. In the following an infrared link implemented accordingto the well known IrDA (Infrared Data Association) protocol stack willbe used as an example, and thus in the example the transceivers 4 and 5are optical transceivers.

FIG. 2 shows an example of the hardware configuration of the infraredtransceiver 4 and its relating control circuitry 6. The infraredtransceiver 4 includes a transmitter diode 7, which will typically be aninfrared light emitting diode, and a receiver diode 8, which willtypically be a photo diode. The transceiver also contains a pulseencoder and a pulse decoder. The control circuitry 6 is normallyimplemented in an ASIC, which could also contain other control circuitsfor the mobile telephone 1. One part of the control circuitry 6 is theIR block 9. This block converts a byte stream into a pulse train fortransmission via the pulse encoder and the transmitter diode 7, and italso converts the pulses received via the receiver diode 8 and the pulsedecoder into a byte stream. The data, i.e. the pulse train, from the IRblock 9 to the transmitter diode 7 are transferred via the line TX,while data from the receiver diode 8 to the IR block are transferred viathe line RX.

Power to the infrared transceiver 4 is supplied from a battery (notshown), and, as shown, power may be supplied separately to the receiverand transmitter parts of the transceiver. In the figure the power to thetransmitter part is supplied through the switch 10 that may becontrolled from the control circuitry 6.

In portable devices it is important to improve the standby time oroperating time between each recharging of the battery, and thus it isalso important to reduce the power consumption of the device. One way todo this is to reduce the power consumption of the infrared transceiver4, because typically it will be inactive for long periods. Two powersaving solutions are well known. One is by means of a Shutdown (SD)signal from the IR block 9 to the infrared transceiver 4. This signalcan put the entire transceiver in a shutdown mode in which both thereceiver and transmitter parts are disabled. When SD is active thetransceiver is switched off and consumes almost no power. When SD isinactive the transceiver is operational, and it can receive and decodeinfrared light pulses.

The other power saving solution is to switch off the power supply to thetransmitter part by means of the switch 10. In this way the transmitterpart is switched off completely, while the receiver part can still beactive, provided the SD signal is inactive. This means that thetransmitter part only needs to be switched on when data are actually tobe transmitted from the transmitter, and since the control circuitry 6controls the data transmission as well as the power switch 10, it iseasy to switch off the transmitter part of the transceiver 4 as soon asit is not needed for transmission of data. However, this is not possiblefor the receiver part, because normally the control circuitry 6 does nothave any knowledge of when data can be expected from the othertransceiver 5. Therefore, the SD signal normally has to be inactive allthe time to ensure that the receiver part of the transceiver is ready toreceive data at any time, in case such data should arrive. As will beexplained later, this is especially important because some types of dataare only sent once.

The fact that at least the receiver part of the transceiver must beswitched on all the time means that the transceiver continuously has acertain power consumption. Some typical figures are that virtually nocurrent is drawn when SD is active, i.e. the whole transceiver is shutdown, while a current in the range from 300 μA to 1 mA is drawn when thetransmitter part as well as the receiver part are switched on. When onlythe receiver part is switched on, i.e. power to the transmitter switchedoff while SD is inactive, the current may be reduced by approximately25%, but still the remaining consumption is considerable.

In the following the word transceiver will be used to describe not onlythe transceiver hardware described above, but also the softwarecontrolling the hardware.

As mentioned above, the transceivers 4 and 5 in this example make use ofthe IrDA protocol, and the data are transmitted according to the serialinfrared procedure IrDA-SIR (IrDA Serial InfraRed) of this protocol.According to this protocol a transceiver can be in a primary mode or ina secondary mode. In the primary mode the transceiver actively searchesfor other transceivers of the same type, normally because it hasinformation to transmit. In the secondary mode a transceiver onlylistens for a transceiver in primary mode to contact it. Normally, thereare long periods with no data transmission, and both transceivers 4, 5will thus be in the secondary mode.

If, for example, data are now going to be transmitted from the computer2 to the mobile telephone 1, the system will enter the IrDA DiscoveryMode and the transceiver 5 will change to the primary mode, thusbecoming a primary transceiver. The primary transceiver will search fora secondary transceiver by transmitting a series of discovery frameshaving start characters in front of them. In IrDA Discovery Mode theprimary transceiver will repeat the series of discovery frames with aninterval or period which is typically set to 3 seconds although othervalues are possible as well. In order to ensure that the secondarytransceiver is able to detect the discovery frames, the discovery framemust include at least some bits which are different from the situationwhere no data are sent. Normally, a binary “1” corresponds to “no lighttransmitted”, and a binary “0” corresponds to “light transmitted”, andthus the discovery frame must include at least some “0” bits. This isachieved in that the discovery frame starts with 10 XBOF characters,which will be explained below. The discovery frames are sent with a bitrate of 9600 baud.

In IrDA Discovery Mode a sequence of e.g. six, eight or 16 discoveryframes, each starting with 10 XBOF characters, is transmitted from theprimary transceiver every period. However, this is not the case inanother mode, i.e. the IrDA Ultra Mode, which is used for example whensending a so-called vCard (business card). This case is importantbecause the information is only sent once, and there is no option forconfirmation or retransmission. Thus the 10 XBOF characters must bedetected the first time. Otherwise, the frame format is the same as inIrDA Discovery Mode.

In IrDA-SIR 9600 the XBOF character has the value 0xFF, but some olderdevices use the value 0xC0 and to ensure compatibility with thesedevices this value should also be supported. When using SIR thecharacters are transmitted in an asynchronous serial format with theparameters 1 start bit (“0”), 8 data bits, no parity bit and 1 stop bit(“1”), which is a total of 10 bits. The bits in the character aretransmitted from the least significant bit (LSB) to the most significantbit (MSB). Thus the value 0xFF is exchanged as 0, 1, 1, 1, 1, 1, 1, 1,1, 1, while the value 0xC0 is exchanged as 0, 0, 0, 0, 0, 0, 0, 1, 1, 1.Note that the 0s result in light pulses while 1s do not. 1s can thus notbe differentiated from silence, so the detection relies on the 0s.

As mentioned, 10 XBOF characters are sent before the actual data packet,and thus the actual bit stream for the XBOF character 0xFF looks likethe sequence below, where it should be noted that the five initial 1srepresent the end of the (probably very long) silence period before thefirst character in the discovery frame.1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1,1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, . . .

If the character 0xC0 is used, the sequence will instead be:1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 1, 1,1, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, . . .

This is also illustrated in FIGS. 3 and 4.

The purpose of the start characters is to “wake up” the receiver toensure that it will actually receive the following data packets. Thereis no information content as such in the start characters.

Thus according to the invention the receiver—when it is in the secondarymode waiting for another transceiver to send a discovery frame—iscontinuously switched on and off in order to save power. This means thatthe receiver is shut down for certain intervals. The receiver just hasto be switched on often enough and long enough to be able to detect atleast one of the zeros in the ten XBOF characters. Then if a zero, andthus a start character, is detected, the receiver is maintained on ofcourse as long as data still arrive from the other end.

At 9600 baud the duration of every bit is 1/9600=104.2 microseconds.Thus an XBOF character (10 bits) takes 1.042 milliseconds, and at leastone of these ten bits is a zero. 10 XBOF characters take about 10.42milliseconds to be transmitted. Thus, theoretically, it is sufficient ifin any period of 10.42 milliseconds the receiver is active in a periodof 1.042 milliseconds. This period is called an eye period.

In practice, however, it is more safe to take into account thepossibility of enabling the receiver in the middle of a zero bit andthereby missing it. Thus the eye period can be extended to 1.146milliseconds (11 bits), and similarly the repetition period can bereduced a little bit. Practical tests have shown that a repetitionperiod of 9 milliseconds with an eye period for the receiver of 1.4milliseconds is completely secure, but values closer to the theoreticalvalues mentioned above are probably possible.

The principle is illustrated in the graph of FIG. 5. A illustrates thesignal SD in the transceiver 4 when this transceiver is in the secondarymode waiting for the other transceiver 5 to contact it. It will be seenthat in each period T_(P) the signal SD is low (i.e. not active) only inthe eye period T_(e). B correspondingly shows that the receiver part ofthe transceiver 4 is switched on, i.e. ready to receive incoming opticalsignals in the eye periods T_(e), while it is shut down in the rest ofthe period. C shows a discovery frame sent from the other transceiver 5,and finally D shows what is actually received in the transceiver 4. Cand D correspond to FIG. 3, i.e. the XBOF character has the value 0xFFand the zeros are represented by the black lines. The period T_(P) isselected to be shorter than, but close to, the duration T_(x) of the tenXBOF characters, and the eye period T_(e) selected to be longer than,but close to, the duration of one XBOF character.

At the time t₁ a discovery frame from the transceiver 5 begins, and inthe shown example the first four zeros are not detected because thetransceiver is shut down. However, at the time t₂ the SD signal becomeslow and the receiver is again ready to receive for an eye period T_(e),i.e. until the time t₃. The zero of the fifth XBOF character lies inthis eye period and is thus detected. Although the SD signal wasotherwise expected to be activated again at the time t₃, as is shownwith the dashed lines in A, it is now kept low and the receiver is readyto receive the remaining XBOF characters and the following data. Ofcourse the receiver is now kept in the switched on state as long as dataare exchanged between the two transceivers. A certain time after theexchange of data has been concluded, the receiver returns to the statein which it is only switched on in the eye periods.

It will be seen that independent of the start time of the discoveryframe at least one of the zeros in the ten XBOF characters will bedetected, and this is sufficient to ensure safe reception of thefollowing data bytes.

Since the eye period can theoretically be close to one tenth of therepetition period, the power consumption of the receiver part of thetransceiver in the discovery mode can also be reduced to close to onetenth of the normal power consumption. Although in practice, asmentioned above, the eye period has to be a little bit longer and therepetition period a little bit shorter, the power consumption can stillbe reduced to maybe 12 or 15% of the normal power consumption.

When an infrared interface without this solution is implemented in amobile telephone, the transceiver typically consumes about 10% of thetotal stand-by current of the phone. Therefore, the user will oftenprefer to switch the interface on and off manually to save power. Withthe solution implemented this value can be reduced to maybe 1 or 2%,which means that the transceiver can now be switched on the whole timewithout affecting the stand-by time of the phone very much. When thetransceiver can be switched on all the time, a user interface to switchit on and off is no longer needed, and it can thus be removed from thephone, which gives a simpler design of the phone user interface.

It should be noted that the feature activates itself when there is noother transceiver in range, but also when another device has actuallybeen found but does not contact the transceiver in which the solution isimplemented.

The fact that the transceiver is always on also means that it is alwaysready to receive an electronic business card that is beamed to thedevice. This is important because such a business card is onlytransmitted once. Today an IR interface has to be enabled or switched onfor some time before a business card can be received.

The SD signal can be controlled from either hardware or software, andthus the solution itself can also be implemented in hardware as well asin software, dependent on what is most convenient in a given device.

Although a preferred embodiment of the present invention has beendescribed and shown, the invention is not restricted to it, but may alsobe embodied in other ways within the scope of the subject-matter definedin the following claims. Thus, the invention has been described abovewith relation to an infrared interface operated according to the IrDAprotocol. However, it should be emphasized that any other protocol usinga number of start characters as the beginning of a transmission can beused as well. Further, it is clear that electrical signals on a wiredconnection or radio signals transmitted through a radio link, e.g. ashort-range radio link, can easily be used instead of the opticalsignals without affecting the idea of the invention.

1. A method of establishing a communications link for exchanging digitalsignals in the form of characters between a transmitter and a receiver,said digital signals being transmitted according to the IrDA protocol,wherein the transmitter for initialization of a communications linktransmits a series of start characters, the method comprising the stepsof: controlling the receiver to alternate periodically between a firststate in which characters can be detected, and a second state in whichdetection of characters is not possible, with a period shorter than theduration of the series of start characters, the receiver having a powerconsumption which is higher in the first state than in the second state,and the alternation being controlled such that in each period thereceiver assumes the first state for a time which is longer than theduration of a start character; and establishing a communications link ifa character is detected, wherein the start characters are according tothe IrDA protocol.
 2. The method according to claim 1, furthercomprising the step of keeping the receiving circuitry in the firststate for a longer time when a communications link is established. 3.The method according to claim 1, characterized in that said digitalsignals are transmitted via a wired connection.
 4. The method accordingto claim 1, characterized in that said digital signals are transmittedvia a radio channel.
 5. The method according to claim 1, characterizedin that said digital signals are transmitted via an infrared channel. 6.The method according to claim 1, characterized in that said digitalsignals are transmitted with a data rate of 9600 bit/s.
 7. The methodaccording to claim 1, characterized in that said start character is anXBOF character.
 8. The method according to claim 1, wherein the methodis performed in a mobile telephone.
 9. A communications interface devicehaving receiving circuitry for receiving digital signals, said signalsbeing transmitted according to the IrDA protocol and said signals beingin the form of characters from an external transmitter according to aprotocol defining that a communications link is to be initialized bytransmitting a series of at least a prescribed number of startcharacters, each start character having a specified duration, saidreceiving circuitry being able to assume a first state in whichcharacters can be detected, and a second state in which detection ofcharacters is not possible, and said receiving circuitry having a powerconsumption which is higher in the first state than in the second state,the device comprising: control means adapted to alternate the receivingcircuitry periodically between the first state and the second state witha period shorter than the duration of the series of start characters,such that in each period the receiving circuitry assumes the first statefor a time which is longer than the duration of a start character,wherein the control means is further adapted to establish acommunications link if a character is detected, and wherein the startcharacters are according to the IrDA protocol.
 10. The communicationsinterface device according to claim 9, characterized in that the controlmeans is further adapted to keep the receiving circuitry in the firststate for a longer time when a communications link is established. 11.The communications interface device according to claim 9, characterizedin that said receiving circuitry is adapted to receive the digitalsignals as electrical signals on a wired connection.
 12. Thecommunications interface device according to claim 9, characterized inthat said receiving circuitry comprises a receiver for radio frequencysignals.
 13. The communications interface device according to claim 9,characterized in that said receiving circuitry comprises a receiver forinfrared signals.
 14. The communications interface device according toclaim 9, characterized in that the receiving circuitry is arranged tocommunicate with a data rate of 9600 bit/s.
 15. The communicationsinterface device according to claim 9, characterized in that said startcharacter is an XBOF character.
 16. The communications interface deviceaccording to claim 9, characterized in that the device further comprisestransmitter circuitry.